Why early birds sing early: rebound singing from nighttime suppression can explain dawn chorus

Many birds sing intensely at dawn, a phenomenon known as the dawn chorus, yet its underlying causes remain unclear. In particular, because most research to date has investigated the adaptive functions (i.e., ultimate causes) of the dawn chorus in wild birds, its behavioral and physiological mechanisms (i.e., proximate causes) remain largely unexplored. Here, we investigated the mechanisms of the dawn chorus under controlled laboratory conditions using the zebra finch, a well-established animal model. We found that birds became hormonally aroused in darkness well before dawn, and that the suppression of spontaneous singing by darkness enhanced their intrinsic motivation to sing. This elevated motivation subsequently triggered a rebound of intense singing immediately upon an increase in ambient light. Given the simplicity of this "rebound singing" mechanism and its independence from social interactions and non-light environmental cues, we propose that this mechanism may critically contribute to the dawn chorus in many avian species. Additionally, we found that rebound dawn singing accelerates morning changes in song structure, supporting the existing hypothesis that the dawn chorus functions as intensive vocal exercise to rapidly optimize song performance. Thus, birds may increase their singing motivation before dawn to compensate for the overnight lack of vocal exercise and, ultimately, to enhance their reproductive success. Together, our results provide a mechanistic explanation for the dawn chorus and offer insights into its adaptive function. Significance statementThe intense singing of many bird species in the early morning is known as the dawn chorus and is widely recognized as a prominent biological indicator of daybreak, yet the underlying mechanisms of this phenomenon remain surprisingly poorly understood. We investigated the behavioral and physiological mechanisms of the dawn chorus in captive songbirds and demonstrated that the suppression of spontaneous singing by pre-dawn darkness induces a rebound of intense singing at dawn. We propose that this rebound singing mechanism may broadly underlie the dawn chorus across bird species. We also obtained experimental evidence supporting the hypothesis that the dawn chorus functions as intensive vocal exercise to rapidly optimize song performance after overnight silence.